by Jeff Miller of Artisan Family of Wines (Seven Artisans, Sly Dog Cellars, Red Côte)
This may be something of an esoteric subject, but it’s an important one for anyone growing grapevines, and it’s one that doesn’t seem to get the attention it deserves, even though it’s pretty basic.
When you leave a bud on the vine at pruning time, that bud will generate a shoot. Most shoots have two grape clusters. So each bud you leave on the vine should produce two clusters. This isn’t always going to be exactly the case, but for present purposes it’s close enough.
In deciding how many buds to leave on the vine, the main consideration should be to end up with a “balanced” vine. Each vine grows vegetative growth (i.e., leaves and shoots) and fruit. Too much vegetative growth in relation to the amount of fruit results in “undercropping”. Undercropping is bad because you tend to get a canopy that is dense and shades the fruit and the inner canopy. Aside from the poor effect this has on the amount of sun getting to the leaves and fruit, it has other unwelcome effects. The density of the canopy can make it difficult for sprays to get into the inner parts of the canopy. Undercropping also results in the vegetative growth continuing longer into the growing season than it should. Normally, the vine transitions from sending its resources to the vegetative parts (leaves and shoots) to ripening the fruit around the time of veraison. If there’s a shortage of crop, the vine will tend to continue to grow leaves and shoots later into the season, resulting in less time devoted to ripening the fruit.
The vine is limited in the amount of fruit it will produce by the number of buds. The converse is not true—i.e., the vine has no automatic way of limiting the amount of vegetative growth. The only way that the vine’s vegetative growth is limited is by there being enough fruit to draw the vine’s resources away from leaf growth.
The opposite of “undercropping” is “overcropping”, where there’s too much fruit for the amount of leaves and shoots. Overcropping results in higher yields, but at the sacrifice of grape quality because there is not enough in the way of leaves to obtain flavor-full grapes.
My rule of thumb is that a vine is balanced when the shoots are 4-5 feet long. If shorter, the vine is overcropped; if longer, undercropped.
Since each additional bud left on the vine results in more fruit, and vice versa, one can in theory leave more buds on the vine and get more fruit. So if you have an undercropped vine (too much vegetative growth), you can try to add more fruit at pruning time by leaving more buds.
Unfortunately, it’s not always easy to leave more buds once you’ve established a vine. In a common cordon system, pruning positions are left from a cane from last season. While each pruning position can have, in theory, any number of buds left, two is the most common number. You can leave three to increase the number of buds, but this often results in the top two buds growing out, but the bottom one not. So you end up with two shoots even though you left three buds. This has the undesired effect that the pruning position tends to grow higher into the canopy. This isn’t a disaster, but other things being equal is something to be avoided.
Alternatively, you can leave one bud instead of two if your vine last year was overcropped. This is not such a bad thing to do, but it will certainly lower your yield. In Napa, because of its higher grape prices, you can probably leave one bud and still have a viable crop. But in most areas of the State, prices probably won’t support the yield you would obtain from leaving just one bud.
If you need more buds and you have the room and money, you can split your canopy. So where you had one fruiting zone, you now have two, with twice the number of buds. This is a good way to deal with undercropping, and opens up the canopy to boot. Unfortunately, the costs of splitting a canopy are significant.
Another way to deal with the problem of undercropping is minimal pruning, a system that creates a hedge-like canopy with a very large number of buds. The vine tends to self-establish the proper amount of vegetative growth in relation to fruit in a vigorous site. This system is widely used in Australia, though has been slow to be adopted here, really for no good reason. I have a small vineyard at my house which was much too vigorous, and I was able to much improve the quality and quantity of fruit by converting to minimal pruning. It’s really a very good system for sites with excessive vigor, which is a common situation through California.
The object of winegrape farming should be to produce grapes that are packed with flavor and color, while being balanced in other ways (i.e., neither too little or too much acidity). In order to accomplish that, you need a balanced vine. A balanced wine is one where neither vegetative growth or fruit crowd out the growth of the other. And essential to that balance is that the vine have the right number of buds, which in turn results in the right amount of grape clusters.
Unfortunately, the idea has taken hold that lower yields give better quality. While this is half true (if the vine is overcropped lower yields will give better quality), it is only half true. With a balanced vine one should leave well enough alone, even if it’s producing a prodigious amount of fruit. If the vine is undercropped, it’s nuts to reduce its fruit even more. You’re just going from moderate to severe undercropping. You end up with lower fruit quality, and lower quantity as well. Yet the erroneous concept that lowering yields always improves quality is, sadly, widely believed, and certainly does more harm than good.